1. Field of the Invention
This invention pertains generally to detecting attempts to bypass motion detectors, and more particularly to detecting, at power up of a motion detector, whether the motion detector has been masked.
2. Description of the Background Art
Motion detectors are widely used in alarm systems. State of the art motion detectors typically employ dual sensing technology, such as a microwave Doppler sensor combined with a passive infrared sensor (PIR), coupled with processing software. In most instances, the PIR sensor is the primary sensor and the microwave sensor is used as a secondary sensor to confirm a detection event from the PIR sensor. While the technology is reliable for detecting alarm conditions based on various sensed conditions, it is still possible to defeat a dual sensor motion detector by "masking" the PIR sensor. It is generally understood in the art that the term "masking" refers to placing a stationary object in front of a sensor, covering the sensor with a substance such as tape or paint, or the like. Even placement of a plate of glass or spraying clear varnish or hair spray over an infrared sensor window can be an effective mask. Most often, the PIR sensor is the target of masking since infrared signals are line of sight whereas microwave signals penetrate and bounce off of objects.
Understandably, mask detection is important if high levels of security are to be maintained at all times and various approaches to mask detection have thus been developed. The simplest is to monitor PIR activity and declare a mask condition if loss of activity occurs for a predetermined period of time, although this method is prone to false mask detects since an empty room will cause a mask condition to be indicated. Another approach is to detect a mask condition during the actual act of masking. In dual sensor detectors employing a microwave Doppler sensor, high level microwave signals are generated when a person or moving object comes into close proximity of the sensor. Therefore, items can be readily detected by a microwave Doppler sensor when they are moving into a position that will block the sensor. Unfortunately, however, once moved into position, a stationary object essentially becomes invisible to a microwave Doppler detector. Another approach is to use a near-infrared emitter/detector pair which looks for a reflected beam. A high reflected signal level would indicate a mask condition because of an object being placed in close proximity. However, this approach is costly and has a relatively high power consumption level.
Therefore, the most reliable approach to mask detection without incurring additional costs in price or power is to use the microwave Doppler sensor to detect close-up events; that is, movement to within approximately eighteen inches of the microwave Doppler sensor. Upon detection of the close-up event, a PIR detection window is opened. If PIR activity is detected during this window, then the mask detection routine ends. Otherwise, if no PIR activity occurs during that time period, a mask condition is declared.
A serious threat to security still exists, however, when using microwave-based mask detection, since this technology is dependent upon seeing the actual act of masking. Therefore, such technology cannot detect a mask if power is removed from the detector, such as, if a detector loses power while a sensor is masked, or the system is powered down during the daytime, or someone masks the sensor during a power outage. In any of those cases, since the masking has already occurred, the sensor will not give an indication that masking has taken place when it is powered up again. Therefore, a need exists for a system and method for detecting that a sensor has been masked without causing the sensor to declare a false masking condition when power loss occurs in an empty building. The present invention satisfies that need, as well as others, and overcomes the deficiencies found in conventional technology.